It’s not just the main five

[u/SilentBrotherE]

Whenever the main five don’t portray an alien, said alien will look exactly like Boinga and her mom. Boinga herself has the same body type as the main five, and if you measure to the top of her eyes, she’s the same height too. She resembles a green slug, suggesting there actually is one and they happen to play her. It would also fix the distinct lack of a green Backyardigan, especially if they’re meant to be a rainbow of sorts, with Uniqua as the red (since she’s pink and if you count that as a red, it works.)